- long in the tooth
This phrase, it seems, can be traced back to St. Jerome, who referred to it as a common saying in his introductory remarks to the Epistle to the Ephesians in his translation of the New Testament: "Equi donati dentes non inspiciuntur." A rather mangled literal translation would go something like this: "A given horse's teeth are not inspected." This is evident from parsing the original Latin sentence: Equi (masculine genitive singular) donati (perfect passive (supine) masculine genitive singular) dentes (masculine accusative plural) non (negative adverb) inspiciuntur (3rd person plural present passive indicative). It is likely that English versions are translations of this original Latin; furthermore, the Latin form seems to explain the use of "given" (geuen) in the 1546 version.
Horses' gums recede as they age making their teeth appear longer (hence the term, "long in the tooth"). Inspecting the teeth of a horse given as a gift was considered ungrateful. It would mean that recipient is trying to see if the horse is old (undesirable) or young (more desirable).
The substitution of "gift" for "given" occurred in 1663 in Butler’s Hudibras, because the iambic tetrameter required a shortening:
- He ne’er consider'd it, as loth
- To look a Gift-horse in the mouth.